Extreme Ultra-Violet Lithography (EUVL) is a leading emerging technology for 13 nm mode and beyond for the production of Micro Processing Unit and Dynamic Random Access Memory (MPU/DRAM) integrated chips. Presently, EUVL scanners which produce these Integrated Chips (ICs) are being produced on a small scale to demonstrate this new technology. The optics systems, which include reflective optical elements, are an important part of these scanners. As EUVL development continues, the specifications continue to become more stringent for the optics system parts.
In EUVL scanners, the optical elements are exposed to an intense extreme ultraviolet (EUV) radiation. Some portion of the EUV radiation used in EUVL systems is absorbed by the reflective coatings on the optical elements of the systems, which results in the heating of the top surface of the optical element by the impinging radiation. This causes the surface of the optical element to be hotter than the bulk of the optical element and results in a temperature gradient through the optical element. In addition, in order to image a pattern on semiconductor wafers, the surface of the optical element is not uniformly heated and a complex temperature gradient is formed through the thickness of the optical element, as well as along the optical element surface receiving the radiation. These temperature gradients lead to a distortion of the optical element, which in turn leads to smearing of the image being formed on the wafers. The low thermal conductivity of materials used in optical elements in the projection systems of EUVL scanners, their large size, and the requirement of operation in vacuum, inhibit efficient heat transfer and removal. It is expected that the difficulties of heat dissipation will be exacerbated by the increased optical element sizes and the increased power levels that are anticipated to meet the demands of future EUVL developments.